Procedure modified by George Lisensky and Jacob Horger, Beloit
College, from the Inverse Opal Photonic Crystals Laboratory Guide by R. Schroden and
N. Balakrishnan, University of Minnesota MRSEC, 2001.
A sol-gel synthesis using tetraethylorthosilicate and close-packed polymethylmethacrylate spheres as a template yields a dimensionally ordered porous silica solid.
Use ethanol to wet a filter paper in a small Buchner funnel connected
to a water aspirator.
Place 2.0 gram of dry polymethacrylate spheres in the funnel and spread
them out to evenly cover the entire filter. Turn on the water aspirator
and slowly wet the entire surface with a well mixed solution of 2.0 mL
100% ethanol, 3.0 mL tetraethylorthosilicate, 1.5 mL pure water, and 0.5
mL conc hydrochloric acid. Try to wet all the spheres before running out
of liquid, but if not all of the polymethylmethacrylate is coated the
yield will simply be smaller. On the other hand, once all the spheres
are wet, stop adding liquid or the product may be too dense. Aspirate
for 20 minutes to speed drying.
Transfer the product to combustion boats.
Place the combustion boats into the quartz liner of a tube furnace
(or a ventilated box furnace) in a hood. Ramp the temperature at 2 degrees
C/minute from room temperature to 300 degrees C to complete the silica
formation. Hold at 300 degrees C for 2 hours. Ramp the temperature at
2 degrees C/minute to 550 degrees C to decompose the polymethylmethacrylate
spheres. Hold at 550 degrees C overnight (ten hours). Cool the oven to
room temperature.
High quality samples will be apparent by their opalescence.
Properties
What color is your product in air? (The index of refraction of silica is 1.460 and of air is 1.000)
Add a drop of ethanol to a small portion of your product. What color do you observe? (The index of refraction of ethanol is 1.360)
Add a drop of toluene to a small portion of your product. What color do you observe? (The index of refraction of toluene is 1.496)
Based on the sphere size used as a template, predict the absorption wavelength for each of these. How do you need to modify the equation for closed-packed spheres for the inverse material?
Characterization with a Scanning Electron Microscope
Samples are deposited on conducting graphite tape and gold coated.
The coated samples are placed into an electron microscope.
Zooming in on a sample of monodispersed polymethylmethacrylate spheres.
What is the diameter of the spheres?
Larger movie in a new window
Zooming in on a sample of monodispersed polymethylmethacrylate spheres.
What is the diameter of the spheres?
Larger movie in a new window
Zooming in on a sample of inverse opal photonic silica crystal. What
is the diameter of the holes?
Larger movie in a new window
Flying over the surface of a sample of inverse opal photonic crystal
(2000x magnification.)
